The present invention relates generally to optical fiber waveguides. More particularly, it concerns methods and structures for efficiently coupling optical fiber waveguides having the same general cross sectional geometry but with cross sectional areas differing in size.
Optical fiber waveguides are well suited for high data rate transfer in both wide area and local area networks. Optical fibers typically include a core formed from a high purity glass-like silica with a glass-like silica cladding layer surrounding the core. The indices of refraction of the core and cladding are controlled during manufacture so that the index of refraction of the core is higher than that of the cladding to confine light energy to the core as it is propagated along the fiber. As compared to prior wire systems, optical fiber waveguides offer small diameter high data rate paths that are relatively immune to interference in electromagnetic environments.
In creating an optical fiber network, such as a telecommunications system, it is necessary that discrete optical fibers be efficiently coupled to one another. One common method of coupling, known as butt coupling, involves cleanly severing the to-be-coupled end of each optical fiber along a plane transverse to the long axis of the fiber. The two severed ends are then joined and maintained together in a core-to-core, cladding-to-cladding relationship so that light from the core of one of the fibers will be transferred into the core of the other fiber. Because the diameter of the cores can be relatively small, as low as 2-3 microns in the case of single-mode optical fibers, it is difficult to align the fibers in an optimum coaxial relationship. Moreover, any off-axis alignment error in the end-to-end butt coupling can result in significant light loss. In addition, the diameter dimensions of both the core and the cladding and their respective cross sectional areas can vary along the length of a fiber, from one fiber to another fiber fabricated by the same manufacturer, and from manufacturer to manufacturer. Potential dimensional variations can make efficient butt coupling difficult. For example, light energy can be lost when attempting to couple light from a relatively large diameter core to a relatively small diameter core, the light loss being related to the difference in cross sectional area between the two cores. Accordingly, a need arises for effecting efficient coupling between optical fibers in such a way that dimensional differences between the coupled fibers can be accomodated.